TC1313 datasheet(21/32 Pages) MICROCHIP

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DS21974B-page 21

TC1313

For example, for a 3.6V input, 1.8V output with a load

of 400 mA, the efficiency taken from Figure 2-7 is

approximately 84%. The internal power dissipation is

approximately 137 mW.

5.6.2

LDO OUTPUT (VOUT2)

The internal power dissipation within the TC1313 LDO

is a function of input voltage, output voltage and output

current. The following equation can be used to

calculate the internal power dissipation for the LDO.

EQUATION 5-7:

The maximum power dissipation capability for a

package can be calculated given the junction-to-

ambient thermal resistance and the maximum ambient

temperature for the application. The following equation

can be used to determine the package’s maximum

internal power dissipation.

5.6.3

LDO POWER DISSIPATION

EXAMPLE

5.7

PCB Layout Information

Some basic design guidelines should be used when

physically placing the TC1313 on a Printed Circuit

Board (PCB). The TC1313 has two ground pins,

identified as AGND (analog ground) and PGND (power

ground). By separating grounds, it is possible to

minimize the switching frequency noise on the LDO

output. The first priority, while placing external

components on the board, is the input capacitor (CIN1).

Wiring should be short and wide; the input current for

the TC1313 can be as high as 800 mA. The next

priority would be the buck regulator output capacitor

(COUT1) and inductor (L1). All three of these

components are placed near their respective pins to

minimize trace length. The CIN1 and COUT1 capacitor

returns are connected closely together at the PGND

plane. The LDO optional input capacitor (CIN2) and

LDO output capacitor COUT2 are returned to the AGND

plane. The analog ground plane and power ground

plane are connected at one point (shown near L1). All

other signals (SHDN1, SHDN2, feedback in the

adjustable output case) should be referenced to AGND

and have the AGND plane underneath them.

FIGURE 5-1:

Component Placement,

Fixed-Output 10-Pin MSOP.

There will be some difference in layout for the 10-pin

DFN package due to the thermal pad. A typical fixed-

output DFN layout is shown below. For the DFN layout,

the VIN1 to VIN2 connection is routed on the bottom of

the board around the TC1313 thermal pad.

FIGURE 5-2:

Component Placement,

Fixed-Output 10-Pin DFN.

Input Voltage

VIN =5V ±10%

LDO Output Voltage and Current

VOUT =3.3V

IOUT = 300 mA

Internal Power Dissipation

PLDO(MAX) =(VIN(MAX) – VOUT2(MIN)) x IOUT2(MAX)

PLDO = (5.5V) – (0.975 x 3.3V))

x 300 mA

PLDO = 684.8 mW

LDO

IN MAX

()

OUT2 MIN

()

–

() I

OUT2 MAX

()

Where:

PLDO = LDO Pass device internal power

dissipation

VIN(MAX) = Maximum input voltage

VOUT(MIN) = LDO minimum output voltage

TC1313

+VOUT1

PGND

+VIN1

AGND

+VOUT2

COUT1

CIN2

COUT2

CIN1

PGND Plane

AGND Plane

AGND to PGND

+VIN2

* CIN2 Optional

- Via

+VOUT1

PGND

+VIN1

AGND

+VOUT2

COUT1

CIN2

COUT2

CIN1

PGND Plane

AGND Plane

AGND to PGND

PGND

CIN2 Optional

+VIN2

TC1313

- Via

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